A serious problem in industrial water systems, especially in cooling water systems, industrial evaporators, and boilers is the deposition of scale, particularly scale-forming salts such as certain carbonates, hydroxides, silicates and sulfates of cations such as calcium and magnesium from aqueous solutions onto heat transfer surfaces. Much of water used in cooling water systems is supplied by the rivers, lakes, ponds, or the like and contains various amounts of scale-forming salts. In cooling tower systems, the cooling effect is achieved by evaporation of a portion of the circulating water in passing over the tower. Because of the evaporation which takes place in cooling, the solids in the water become concentrated. Moreover, because of the inverse solubility of calcium carbonate, calcium sulfate and other hardness salts, the problem of the formation of water-insoluble scales on the heat transfer surfaces is intensified.
Various organic phosphonates have been considered for use in scale control. For example, U.S. Pat. No. 3,336,221 describes a method of inhibiting the precipitation of scale-forming salts in an aqueous system comprising adding to said system compounds having a methyl phosphonic acid bonded to a nitrogen atom such as amino tri (methylphosphonic acid) or phosphonomethyl ethanolamines. Unfortunately, these compounds are not chlorine resistant and degrade in the presence of free chlorine which commonly used as a disinfectant or biocide in many aqueous systems. U.S. Pat. No. 3,214,454 teaches use of certain acylation products of phosphorous acid (e.g. hydroxyethylidene diphosphonic acid) for scale control. Unfortunately, various phosphonates including hydroxyethylidene diphosphonic acid and amino tri(methylphosphonic acid) are very sensitive to calcium hardness and prone to form calcium phosphonate precipitates.
U.S. Pat. No. 3,474,133 discloses that certain organo-phosphono-amine oxide compounds can be prepared by oxidizing organo-phosphono amine with a suitable oxidizing agent. For instance ethanol bis(dihydrogen phosphonomethyl) amine can be reacted with H.sub.2 O.sub.2 to yield ethanol bis(dihydrogen phosphonomethyl) amine oxide (i.e. HOCH.sub.2 CH.sub.2 N(O) (CH.sub.2 PO.sub.3 H.sub.2).sub.2); and tris(dihydrogen phosphonomethyl) amine can be reacted with H.sub.2 O.sub.2 to yield tris(dihydrogen phosphonomethyl) amine oxide (i.e. ON(CH.sub.2 PO.sub.3 H.sub.2).sub.3). It is disclosed that the organo-phosphono amine oxides have utility in practically all fields of organic chemistry wherein their acidic or salt and/or amine oxide properties can be utilized; and the various utilities indicated for the compounds in such fields include utility as sequestering or chelating agents, water treating agents, stabilizers for peroxy compounds and corrosion inhibitors. In particular, the acids and water soluble salts of the tris(phosphono lower alkylidene) amine oxides are reported to exhibit the property of being effective sequestering agents for metal ions in alkaline mediums. For example, the penta sodium salt of tris(dihydrogen phosphonomethyl) amine oxide is reported to sequester calcium ions in alkaline media in over a mole per mole basis. These tri(phosphono lower alkylidene) amine oxide compounds are considered very sensitive to calcium hardness and they are prone to form calcium phosphonate precipitates.
There is a continuing need for safe and effective water treating agents which can be used to control scale formation, or to control corrosion particularly when substantial calcium is present in the system water. Water treating agents of this type are especially valuable if they remain stable in the presence of free chlorine which is commonly used in aqueous systems as a biocide or disinfectant.